1. Field of the Invention
The invention relates to the field of illumination devices for optical valves. Such devices are known as light boxes. Optical valves are devices that carry out a spatial and/or temporal filtering of light. The most widely known light boxes are formed by matrix arrays of liquid crystals controlled by matrix arrays of electrodes.
2. Description of the Prior Art
A description of the prior art in this field is given in a report on a communication to the 13th International Colloquium on Research in Display Devices, pages 249-252, September 1993. The report is entitled "A novel projection single light valve high brightness HD color projector".
The author of this article P. JANSSEN (Philips Laboratories, Briarcliff, USA) describes a new valve projector and summarizes the prior art in this field.
Thus, he states (page 249, column 2) that there are two types of single-valve projectors, namely spatial projectors and temporal projectors. With regard to spatial projectors:
"Mosaic color filter: The image of a direct view color display is projected on the screen. Three color pixels represent one resolution element. Unless the pixels are superimposed through some depixellation scheme, still more pixels are needed for a similar quality impression, which further reduces pixel aperture ratio and light transmission. Two-thirds or more of the light is lost in the color filters."
With regard to sequential valves, the following is explained:
"A single light valve is flooded sequentially with red, green and blue light, for instance by spinning a color filter in the illumination path (3). As in the case of mosaic filter projection, the color wheel transmits, on average, only one-third of the optical power."
The author explains that, in both these types of valve, only one-third of the emitted power is available at output. He then describes the principle of a new single-valve projection device with reference to FIG. 1 of the report.
The white light from a small arc lamp is distributed (in a manner known in the prior art) through dichroic filters. From the different colored beams created, an optical system converts the beams into red, green and blue rectangular bands on the valve. These bands are separated by dark spaces. A prism in the optical path acts as a vibrating plate with parallel faces for the shifting, by rotation, of the colored bands on the valve. Whenever a colored band reaches a boundary of the valve, for example the bottom boundary, it is redirected upwards from where it continues it downward movement. The process of permutation of the colors is illustrated in greater detail in FIG. 2 of the report. According to FIG. 2 and the explanations given with respect to the associated electronic circuitry for the control of the valve, it can be understood that the valve is divided into three horizontal rectangular parts, each part corresponding to a colored rectangular beam.
Each of the parts is illuminated alternately, through the rotation of the prism, by a red beam, then a green beam and then a blue beam. When a part is illuminated by the red light for example, the pixels of these parts are activated to filter the red light and give the red part of the image to be projected. Similarly, for the blue and the green. Thus, for an image refreshed 60 times per second, each of the three bands of the screen is refreshed 180 times per second, 60 times with a red image, 60 times with a green image and 60 times with a blue image. Dark parts between the three colored light beams correspond to the time taken by the crystals of the valve to change their state between two successive commands, namely about 3 milliseconds.
Although the technical arrangement described in this document has advantages, notably as regards the luminous power transmitted by the valve, it still has a few drawbacks that shall be referred to here below.
The light source and the associated optical system for shaping the light beam form a fairly bulky unit that notably cannot be used to obtain compact head-up color visors.
Since the light source is a single source, its reliability directly affects that of the display. Finally, whether the light source is an arc lamp, an incandescent source or a fluorescent-lamp-based source with a large surface area, there is always one specific additional drawback pertaining thereto.
Each of these points shall be developed at greater length here below.
An illumination device with a small-sized light source such as the one described in the document referred to has a depth in a direction perpendicular to the plane of the valve that is directly proportional to the size of the diagonal of the valve as explained here below with reference to the appended FIG. 1.
The typical structure of the system for the shaping of a light beam is shown in FIG. 1. A small-sized light source 1 has on one side a spherical mirror 2 sending light emitted by the source 1 to a condenser 3 located on the other side of the source which collimates light beam coming from the reflector 2 and the light beam coming directly from the source 1 in a direction perpendicular to the plane of an optical valve 4.
The value "D" of the diameter of the condenser 3 is approximately equal to the length of the diagonal of the optical valve 4. The condenser 3 and the reflector 2 are positioned on either side of the source 1, the condenser 3 being removed from the source 1 by a distance equal to the focal distance "F" of the condenser. The minimum value of F is approximately equal to the diameter D, given the usual limitation of aperture of the optical systems.
Thus, the minimum value of the shaping system corresponds roughly to a cylinder with a diameter D and a length greater than D, in overlooking the thickness of the condenser 3 and of the reflector 2.
The other shaping systems using for example an elliptical reflector and a parabolic reflector lead to a longitudinal dimension of the same order of magnitude.
In the case of the document referred to, the useful height of the valve is reduced by one-third. This corresponds to a diminishing of the length of the diagonal but it is necessary to add the depth of the rotating system that deflects the light beams and the depth of the system for the anamorphosis of the light beams.
The light source 1 used is either a filament-based incandescent lamp or an arc lamp as in the document referred to. The incandescent lamp has several drawbacks:
the luminous efficacy is low; PA1 the fall in the luminous level by variation of the current in the lamp is accompanied by a chromatic drift towards the red and a drop in luminous efficacy. PA1 the variation in luminance is hard to achieve electrically; PA1 the efficient discharge of calories prohibits certain operating positions. PA1 the concentration of luminous power on a small radiating surface leads to a high concentration of heat which it is difficult to discharge; PA1 the radiation given is not monochromatic; it is generally white; PA1 the reliability of the system of illumination is highly dependent on the reliability of the lamp. PA1 the luminance of the radiating surface is low; PA1 the radiating surface is great; PA1 the radiated light flux is low. PA1 the available light flux is low; PA1 the direction of radiation given is omnidirectional. PA1 a sufficient level of illumination on the optical valve; PA1 sufficient spatial homogeneity of illumination; PA1 a direction and a divergence of light flux compatible with the valve and the projection system; PA1 maximum luminous efficacy for the color considered; PA1 minimum volume, especially small depth; PA1 operation in every position; PA1 maximum gradation of the light level; PA1 maximum operating security.
The arc lamp has other drawbacks:
These two technologies have other drawbacks in common:
For the retro-illumination of the optical valves in direct viewing mode, the characteristics of radiation of the retro-illumination beam are not the same. Another type of light source is used: the fluorescent tube.
Its drawbacks, for projection systems, are:
For valves in direct viewing mode, several fluorescent tubes are thus used to increase the light flux. The shaping system is based on the use of scattering devices working by reflection transmission.
The luminous yield of such a system is low.
Furthermore, this type of system not appropriate for projection because:
Should the image be given by the display be monochromatic, for example green, as is the case for a head-up visor, a filter could be interposed between the source and the valve to eliminate the undesirable light rays which consequently reduce the luminous yield.
If the image to be given is a color image, it is synthesized either spatially or temporarily from three monochromatic images, red, green and blue, as explained in the introduction to the document referred to.
The spatial synthesis can be done either by using three optical valves or by using a single optical valve.
For the single-valve spatial synthesis, to each pixel there is added a red, green or blue optical filter that reduces the luminous yield. For the three-valve synthesis, the white radiation is split up into three types of radiation, individually red, green or blue having different positions and illuminating each of the valves separately. The optical splitting systems are bulky. Such systems are described for example in the THOMSON CSF patent No. 89.01533 dated Feb. 7, 1989.
For temporal synthesis, a single optical valve is used. Each pixel is retro-illuminated successively by a color beam that is red, green and then blue. The device described in the document referred to exemplifies such a construction.
With reference to the prior art that has just been described, the aims of the invention can be summarized as follows.
The invention is aimed at a light box for an optical valve having:
The invention is also aimed at ensuring that reliability is not related to a flimsy element such as an arc lamp or an incandescent filament bulb. For color displays, it is aimed at the elimination of the device for splitting the beam coming from the source into several colored beams. It seeks to obtain a stable light source that does not require a high degree of calorific discharge. It finally seeks to obtain a device for the illumination of a valve that comprises no moving parts.
According to the invention, the light source for the retro-illumination of the optical valve is formed by several slotted fluorescent tubes of identical colors positioned in parallel with one another. The shaping system has a longitudinal lens for each of the light tubes. Each lens is positioned in parallel to the tube associated with it and distributes the light from this tube on a band of the valve, this band being parallel to the tube and to the lens.